Picking The Right One As Hard As Choosing A Car

January 20, 1991|By Phil Arendt, The Tribune`s Dr. Gizmo.

You`re eastbound on the Eisenhower at Mannheim Road.

It`s 50 minutes to the post office, and your important appointment downtown is in 45. The low-fuel warning light just came on and, on the radio, following these commercial messages, it`s a 45-minute music marathon by the Monkees.

Three days later, following your release from pyschiatric care, it`s time to visit the tire store, where you`ll quickly learn that buying tires makes driving to work look easy. More than 125 brands of tires are manufactured, according to the U.S. Department of Transportation.

Obviously if a tire is flat or damaged, and can`t be repaired, it needs replacement. Another less apparent cause for replacement is wear beyond factory specifications. Tread-warning bars become visible when the tire needs to be replaced immediately. These bars are flat spots that run from one side of the tread to the other.

Another way to determine tire wear is to measure the depth of the tread. Tire tread depth gauges, available at auto supply stores, measure the depth of the tread in 32nds of an inch. A tire worn to 2/32nds of an inch or less needs replacing.

If a gauge isn`t handy, use a penny. Insert the penny, head down, in the tread. If Lincoln`s hairline is visible, it`s time to put that penny toward a new tire.

Tires can be divided into three types: conventional, all-season (which are popular in this area) and snow tires. Many cars come from the factory with all-season tires. If not, carmakers usually can install them as options. Snow tires also can be ordered, but generally are used in parts of the country where streets are not plowed.

What`s best for a car depends mostly on the owner`s driving habits and what the carmaker recommends.

The factory chooses tires that provide the best handling, load capacity, fuel efficiency, ride, puncture resistance and traction for that given model. Before buying tires, consult the owner`s manual, which lists a specification number for tires suited to the car. Or, look at the side of the tire. If you`re happy with the tires already on the car, buy the same kind as indicated by the letters and numbers stamped on the sidewall.

All tires are made of a mixture of rubber and carbon, but the similarities end there. The rubber is strengthened by layers, or plies, typically composed of steel, nylon, fiberglass, aramid, polyester, Kevlar or a combination.

The three most common types of plies are bias, belted-bias and radial. Carmakers most frequently equip new cars with radials because of their strength, flexibility and fuel efficiency. However, a few models come with belted-bias tires.

Bias plies run at 32-degree angles from each sidewall. Belted-bias tires are similar, but have additional plies around the circumference to add strength. Radial tires have plies that run at 90-degree angles from sidewall to sidewall, allowing the sidewalls to flex. Also, plies similar to those in belted-bias tires span the circumference of radials.

The tire size, based on the commonly used P-metric system, is stamped on the sidewall in letters and numbers. For example, a 1986 Oldsmobile Cutlass has a tire size of P185/75R14. The ``P`` means the tire is for a passenger car. A light truck tire would have an ``LT`` designation. The ``185`` means the tire is 185 millimeters, or about seven inches, wide.

The next two numbers denote the aspect ratio or profile ratio, which is the relationship of the tire height, or the distance between where the tire mounts on the tim to the tread, to the width, the distance across the tread. For the Cutlass, the ``75`` means the tire is 75 percent as high as it is wide. The lower the number, the smaller the distance from the tread to the rim. Most tires have an aspect ratio of 70 or 75, but it can range from 40 to 80.

A letter listed after the aspect ratio tells the type of ply. ``R`` means the tire is a radial; ``D,`` a bias ply; and ``B,`` a belted-bias ply. The last two numbers identify the size of the wheel on which the tire is mounted. Tires on this Cutlass mount on a 14-inch rim.

To confuse you even more, some tires are measured by a Euro-metric system. This system doesn`t use an alphabetical prefix such as the ``P`` and doesn`t list aspect ratios unless they are less than 70.

Recently, tiremakers have incorporated a speed-rating identification system. Speed-rated tires, generally used on high-performance cars, have an additional letter before the tire ply identification. ``S`` means the tire is capable of sustained speeds up to 112 miles an hour; ``T`` means up to 118 m.p.h.; ``H,`` up to 130 m.p.h.; ``V,`` up to 149 m.p.h.; and ``Z,`` above 149 m.p.h. Conventional tires without a speed rating must withstand speeds of 85 m.p.h.

In addition, the side of the tire lists the manufacturer`s name, name of the tire and the following: